1. Field of the Invention
This invention relates to an automatic focusing device using a video signal obtained from an image sensor.
2. Description of the Related Art
Various methods which have been proposed for automatic focusing devices include a method of using a video signal obtained from image sensing means. In the case of this method, automatic focus adjustment is generally performed either by extracting a high frequency component of a video signal or obtaining a differential value thereof and by shifting the position of a lens in the direction of increasing the absolute value of the high frequency component. The method of this kind has long been known, for example, from an article entitled "Automatic Focus Adjustment of TV Camera by Hill Climbing Method," appeared in "NHK Technical Research Report," 1965, Vol. 17, No. 1 (Whole No. 86) which has described this method in detail.
FIG. 1 of the accompanying drawings shows a typical arrangement of the hill climbing method. Referring to FIG. 1, the illustration includes a lens 1; an image sensor 2 which is arranged to convert an image formed on an image sensing plane into an electrical signal; a preamplifier 3 which is arranged to amplify a video signal produced from the image sensor 2; a processing circuit 4 which is arranged to convert the output of the preamplifier 3 into a video signal in conformity to such standardized specifications as those of the NTSC color system; a band-pass filter (hereinafter referred to as BPF) 5 which is arranged to extract only a high frequency component from the output of the preamplifier 3; a detection circuit 6 which is arranged to detect the output of the BPF 5; a motor driving circuit 7 arranged to drive a lens driving motor 8 on the basis of the output of the detection circuit 6; and the lens driving motor 8 which is arranged to drive and shift the position of the lens 1 for focusing.
In accordance with the above stated arrangement, the image formed on the image sensing plane of the image sensor 2 via the lens 1 is converted into an electrical signal, or more specifically, a video signal. This signal is amplified to a given level by means of the preamplifier 3. The video signal has its high frequency component vary according to the position of the lens 1, that is, in proportion to the focusing state of the lens 1 on an object to be photographed. The high frequency component increases accordingly as the lens 1 comes closer to an in-focus state for the object. The lens 1 is in-focus when the high frequency component of the video signal is at a maximum value. FIG. 2 shows variations taking place in the high frequency component of the video signal in relation to the lens position. The high frequency component reaches a maximum point at a point A. As shown, the high frequency component decreases accordingly as the lens 1 deviates from the in-focus point. Hence, the in-focus state of the lens 1 is attainable by shifting the position of the lens 1 in a direction in which the maximum point of the high frequency component is obtainable and by stopping the lens 1 at that specific point.
However, a camera having an automatic focusing device arranged in the manner as described above has the following drawback: In case that the camera is shaken in the horizontal direction while the lens 1 is set in an in-focus position, i.e., at a point A of FIG. 2, the high frequency component level of the video signal drops to become lower than a level obtained when the lens 1 is in the same in-focus state without any shake of the camera. The reason for this: The image sensor 2 is arranged to accumulate in each of its picture elements an image projected thereon via the lens 1 during one field period (about 1/60 sec in the case of the NTSC system, hereinafter referred to as 1 V). In the event of camera shake or in the case of a panning operation, the whole image formed on the image sensing plane of the image sensor 2 shifts to an extent as much as the moved extent of the camera within one field period. Then, the image component to be accumulated in one picture element when the camera is in repose comes to be accumulated over a plurality of adjacent picture elements. In other words, the image under such a condition becomes as if a video signal is applied to a low-pass filter. As a result, the high frequency component of the signal comes to diminish.
In the event, the lowered high frequency component misleads the automatic focusing device to judge that the lens is out of focus for the object. Then, while the lens is in-focus in actuality, the lens driving motor 8 comes to be driven to actuate the automatic focusing device. In that event, the lens is temporarily brought out of focus. Generally, a video camera, particularly a home or handy-type video camera, is held by hand for photographing. Hand shake thus tends to be introduced to the camera. It has been desired to prevent the camera from being brought out of focus by such camera shake or by a panning operation.
Further, another problem arises when the object moves after the lens is focused on the object. In the conventional automatic focusing device which is arranged as shown in FIG. 1, the variations as shown in FIG. 2 take place in the high frequency component of the video signal only in the case of a single object with a relatively uniform background shown in FIG. 3(a). In cases where the background is complex as shown in FIG. 3(b) or where the object has another object in a different position as shown in FIG. 3(c), the high frequency component no longer comes to vary in such a simple manner as shown in FIG. 2 but varies to have a plurality of peak values as shown in FIGS. 3(d) and 3(e). In such a case, the conventional method of using image information on the whole image plane tends to cause a so-called depthwise confusion in which the lens is focused neither on the object nor on the background. To solve this problem, a gate circuit is inserted in between the preamplifier 3 and the BPF 5 of FIG. 1. The distance measuring range is limited by this arrangement by allowing only such a part of the video signal that corresponds to the focusing zone (a distance measuring frame) set within a specific part of a video image plane as shown in FIGS. 4(a) and 4(b). This is one of varied methods conventionally used for solving the problem of the above stated depthwise confusion.
FIG. 4(a) shows a method wherein the distance measuring frame is not only set on the image plane but also arranged to have the size thereof variable in the middle part of the image plane, so that a focusing action can be carried out appositely to the size of the object. The details of this method are disclosed, for example, in Japanese Laid-Open Patent Application No. SHO 61-18271.
FIG. 4(b) shows another case, wherein the location of the distance measuring frame on the image plane is arranged to be variable upward, downward, leftward or rightward. A focusing zone is thus arranged to be variable by shifting the distance measuring frame by operating a button switch according to the position of the object. The details of that method are disclosed, for example, in Japanese Laid-Open Patent Application No. SHO 57-183186, Japanese Laid-Open Patent Application No. SHO 59-105773, etc.
These methods, however, have presented the following problems: In the case of FIG. 4(a), the method enables the size of the distance measuring frame to be automatically changed in the middle part of the image plane according to the focusing state of the lens. However, this method is effective only in cases where the object is set in the middle part of the image plane. However, unlike a still camera, a video camera is often used for taking a picture of a moving object and tends to be seldom used for taking a picture of an object staying in the middle of the image plane. When the object moves outside of the distance measuring frame during a picture taking operation, the lens is focused on something else.
In the method of FIG. 4(b), the distance measuring frame is shiftable accordingly as the object moves. However, the shift is manually effected by means of a push button switch or the like and is limited to the location of the frame. The camera is incapable of automatically detecting and tracing the movement of the object. Therefore, the operator is required to manually perform a position control operation, which is complex and tends to result in a faulty operation. Since the operator is required to constantly follow the object, such a picture taking operation imposes an excessive burden on the operator. The method, therefore, can hardly be regarded as advantageous.